Direct access storage devices (DASD) are widely used to store data in computer systems. A common class of DASD can be referred to as moving storage devices. This class includes magnetic tape and disk drives, optical disk recording devices and other types of moving media recording devices.
A commonly used moving storage device is the magnetic media disk drive typified by the IBM systems 3350 and 3380.
Data is typically stored on moving storage devices in the form of records. The records for a given device are recorded according to a particular format.
Advances in recording technology lead to changes in recording format. Thus as a new device is developed, a need arises to emulate the format of older devices already in the field on the newer devices. By providing this emulation capability, the installed recording capacity of devices operating according to varying recording formats can be used along side new devices without complicated reprogramming of data processing systems that utilize the data stored.
The prior art is typified by U.S. Pat. No. 4,223,390, entitled "The System and Method for Attaching Magnetic Storage Devices having Dissimilar Track Capacities and Recording Formats", issued Sept. 16, 1980, invented by Bowers et al. This Bowers et al patent is incorporated by reference for the purpose of illustrating the prior art and typical recording formats.
The Bowers et al invention performed a track format level of emulation which suffers several disadvantages. First, the end of the emulated track on a new device was not at a fixed location. Thus the size of the track being emulated on the new device was of a variable length. Further the Bowers et al emulation did not maintain a positional mapping of record on the new device. This lack of positional mapping could cause early reconnections to a channel trying to access data on the disk and waste channel and control unit time.
The Bowers et al invention did reduce the complexity of reprogramming needed in data processing systems accessing the data but paid a considerabale cost in speed of access to data on the track.
Emulation of complicated recording formats, such as count-key-data (CKD), on new devices having different track characteristics highlight the problems associated with emulation. Thus the following discussion of a typical CKD format is provided for the purposes of background information.
A CKD formatted track includes a number of unique fields. The fields include an index field I, a G1 gap, a home address HA field, a G2' gap and for each record Rn a count field RnC, a G2 gap, an optional key field RnK followed by a G2 gap and a data field RnD followed by a G3 gap. The number or recordable units needed to fill the various types of fields and gaps typically varies from one CKD format to another independent of the content of the data field. Also the gaps are often defined as a number of recordable units necessary on a device operating according to the format needed to provide a time window for execution of control programs. Thus if the device doing emulation of that format operates at a different rotating speed or recording density than the original device, the number of recordable units necessary for a gap that emulates the original format could be proportionally different.
In part due to the complexity of emulating complex recording formats, the prior art has been unable to maintain positional mapping for emulated records. Therefore, modern recording devices, which include a rotational position sensing apparatus to most efficiently utilize channel and control unit time and prevent early reconnection of the channel to the recording device, have been unable to rely on that rotational position sensing apparatus for emulated records.